The present invention relates generally to controlling the output of lights. More particularly, embodiments of the invention relate to a method and apparatus that use LEDs as light sensors for detecting light levels in an area or room and for controlling these light levels.
Lighting control circuits are used with electronic dimming ballasts. These ballasts control the output of lights, such as fluorescent lights, that illuminate areas such as rooms, offices, patios, etc.
Traditionally, photocells and photodiodes are used as photo-transducers or light sensors for lighting control systems. A photocell is a device that detects light in a controlled area or room. It then uses information from the light, e.g., illumination level, to adjust light output in the controlled area.
Photocells and photodiodes are wide spectrum sensors and they respond to a spectrum much wider than the spectrum perceived by the human eye. This is acceptable for a variety of lighting control systems including systems operating in areas were the controlled light has the same spectrum all times, e.g., where only fluorescent lights are delivering the illumination. If the spectrum distribution remains the same, the resultant electrical energy is proportional to visible energy or light. Hence, a lighting control system can be adjusted to keep the visible light level constant.
Typically, the light in a controlled area or room has two or more different contributing light sources, e.g., artificial light plus sunlight. For example, the controlled light source could be fluorescent lighting and the variable or “disturbing” source could be the sun, i.e., daylight. Note that for the purposes of discussion, the terms sunlight, daylight and natural light are used synonymously. Similarly, the terms electrically produced light and artificial light are used synonymously. Artificial light would include for example fluorescent light, incandescent light, HID, etc.
Different light sources could have different energy spectrums. For example, radiometric energy spectrum of sunlight is wider than that of electronically produced light such as fluorescent light. Similarly, the energy spectrum of a fluorescent light is different from that of an incandescent light. Also, the human eye perceives only a part of the energy spectrum emitted by all available light sources, e.g., sun light, incandescent light, fluorescent light, etc. Research done on a variety of human subjects shows that the sensitivity of the human eye varies with the lighting level. It is widely accepted by specialists in the field that under daylight conditions the spectral response of the human eye can be approximated by the so-called “photopic curve.” This has a well-known bell shape and ranges from about 460 nm to 680 nm wavelengths, with the peak in the region of 560 nm.
Some research has shown that under poor illumination conditions the human eye changes its spectral sensitivity. Also, low illumination affects different people differently. A new characteristic has been devised for this behavior. It is called the “scotopic curve.” This is centered at about 410 nm and covers the spectrum from about 380 nm to 450 nm. In analyzing its overall behavior, it is perhaps appropriate to say loosely that the human eye can perceive light in the range of 400 nm to 700 nm.
A problem arises because most conventional photo-transducers capture or detect the entire energy spectrum produced by all light sources. Thus, when the photo-transducer transforms the captured light energy into a current, it does not distinguish between different wavelengths of light, i.e., sunlight and artificial light. This conventional design of lighting control systems is based on the assumption that the current represents visible light. Unfortunately, this is a poor assumption. In one known light controller circuit, for example, a current resulting from both natural and artificial light components is interpreted by a subsequent circuit as though it is a current merely resulting from the artificial light contribution. Accordingly, the system dims the artificial lights until the resultant voltage equals a set point or preset illumination level. This is problematic because the resultant voltage is derived from both natural and artificial light components which include non-visible energy, while the preset, illumination level is set according to visible light standards, e.g., 40 foot candles. Consequently, this could result in full dimming of the artificial lights when the incoming daylight provides insufficient illumination for a typical room.
Some circuits use a light filter to allow only the visible spectrum to reach the photo-transducer. For example, an optical filter placed over a photo-transducer can achieve this. This would mimic the photopic curve or visible spectrum. Light sensors using optical filters are more efficient than conventional photocells used without such filters. Optical filters, however, are expensive. These special pickup heads are typically used in some professional applications. Note that the term optical sensor, as used herein, is used to mean a photo-transducer used with an optical filter.
Thus, it is desirable to have an alternative illumination management system that can detect a spectrum of light close to that which the human eye detects.